/Engine/Source/Runtime/Engine/Public/Animation/AnimInstanceProxy.h
Get the index of the specified instance asset player. Useful to pass to GetInstanceAssetPlayerLength (etc.). Passing NAME_None to InstanceName will return the first (assumed only) player instance index found.
int32 GetInstanceAssetPlayerIndex ( [FName](API\Runtime\Core\UObject\FName) MachineName, [FName](API\Runtime\Core\UObject\FName) StateName, [FName](API\Runtime\Core\UObject\FName) InstanceName ) const